This invention relates to direct current (D.C) interrupting apparatus.
Different from alternating current (A.C), since direct current (D.C) has no natural zero point, interruption of large DC currents is difficult. According to one prior art method a commutating circuit including a capacitor is connected in parallel with an interrupter so as to pass commutation current through the contacts of the interrupter in a direction opposite to the load current, that is the current to be interrupted, at the time of circuit interruption, thereby forcibly forming a zero point. Accordingly, it is possible to use circuit breakers designed to interrupt alternating current to interrupt direct current.
However, the operating duty of the DC interrupter is different from that of the AC interrupter in that (1) in the AC interrupter the current value varies after separation of the contacts, whereas in the DC interrupter, a large DC flows until commutating current flows and (2) in the AC interrupter, the rate of current change near the zero point is determined by .omega. I.sub.o (where .omega. represents angular velocity and I.sub.o current) whereas in the case of the DC interrupter it is determined by the commutating current. The interrupting capacity of the AC interrupter is determined by such factors as the peak value and the energy of the current, and the rate of current change near the zero point. As has been pointed out hereinabove since the rate of current change of a DC interrupter is determined by the commutating current, in order to make equal the rate of current change of the DC interrupter to that of the AC interrupter operating at 50 Hz it is necessary to increase the capacity of the commutating circuit. For this reason, it should be noted that the rate of current change is greater in the DC interrupter than in the AC interrupter. Further, the injection energy from the commutating circuit is larger in the case of the DC interrupter. Accordingly, the contacts of the DC interrupter are subjected to more severe conditions than those of the AC interrupter.
Vacuum switches are now believed to be most suitable as DC interrupters because of their high rate of current change near the zero point. Yet, it is necessary to increase their current capacity. To increase the current capacity it is necessary to improve the vacuum tank and the method of parallel operation of a plurality of vacuum interrupters. Parallel operation is advantageous because it is possible to increase the overall interrupting capacity by increasing the number of vacuum interrupters of standard design.
Parallel operation requires current balance between branches. Since the vacuum interrupter has a positive voltage-current characteristic (that is the arc voltage increases with the current) it is considered that it is suitable for parallel operation. However, due to the unbalance in the arc voltages and the difference in the contact separation initiation points it has been necessary to connect current balancing reactors in series with respective vacuum interrupters. In a DC circuit, formation of the current zero point is difficult in a branch carrying a larger current and the rate of current change becomes severe in a branch carrying smaller current thereby resulting in a failure of satisfactory current interruption. Once, the current interruption fails, a large fault would result because the commutation circuit is no longer effective.
Large inductors are generally used as the balancing reactors. However, use of large inductors increases the capacity of the commutating circuit. Accordingly, it is necessary to interlink the inductors in respective branches so as to make the overall inductance equal to zero but cause the inductors to balance the currents flowing through the branches. This arrangement, however, can not improve the interrupting characteristic of the circuit interruptors.